Consumer packaging is used to control the air, moisture, light environments and to protect the integrity of a product. In the case of a food product, such packaging is used to preserve the freshness by providing a barrier to passage of air, water vapor or other contaminants, especially gaseous contaminants. This type of consumer packaging, such as bags and packages, is commonly produced by high-speed packaging equipment from paper, aluminum foil, single and multi-layer films, and the like.
In the case of multi-layer films for consumer packaging applications, it is important that such films have optimum barrier properties and produce a hermetic seal when formed using high-speed packaging equipment, such as horizontal form/fill/seal (HFFS) packaging equipment and vertical form/fill/seal (VFFS) packaging equipment. Such multi-layer films have been developed which employ oriented polypropylene (OPP) as a core layer and typically include one or more additional layers, such as coatings, co-extrusions, laminations, and combinations thereof.
These additional layers are added to the core layer of an OPP film in an effort to improve the barrier properties and sealing characteristics over that obtained with the core layer of an OPP film alone. Additives may be added to these layers to further enhance other film characteristics, such as antiblocking, antistatic slip, and coefficient of friction. When a film is processed on high-speed packaging equipment, the antiblocking and slip characteristics of the film are very important. There may be undesirable adhesion between adjacent film layers, referred to as blocking, when a film is unwound from a roll. The coefficient of friction between the film and high-speed packaging equipment surfaces may be too high. Thus, a film that exhibits good antiblocking characteristics and a low coefficient of friction for good processability on high-speed packaging equipment is desirable.
Also, the sealing characteristics of a film processed on high-speed packaging equipment are very important. HFFS and VFFS equipment, for example, includes sealing operations at various stages. In HFFS equipment, packages are formed by folding the film through a forming box and sealing along the length by hot wheels, hot bars or other devices, and then sealing at both ends in a transverse direction by sealing jaws. Cutting knives located in the sealing jaws are used to separate the individual packages. In VFFS equipment, individual packages are formed by forming the film around a forming collar and the continuous web created thereby is immediately sealed together by a longitudinal sealing jaw. In addition, there is a second sealing operation in VFFS equipment that consists of a combined top-sealing section and bottom-sealing section, with a package cut-off operation in between. The top-sealing section seals the bottom of an empty package suspended from the forming collar while the bottom section seals the top of a filled package. The package cut-off operation separates the individual packages.
In hot seal packaging applications for HFFS and VFFS equipment, the seal is formed by the application of heat and pressure. In cold seal operations, the seal is formed by first applying an “adhesive” to the film area to be sealed, followed by the application of pressure by the sealing jaws.
Uncoated thermoplastic films, such as coextruded OPP films, for example, have higher than desired hot seal temperatures and a narrower than desired heat-sealing ranges for very high-speed packaging applications. Such films often exhibit a tendency to disorient and shrink when they are heated to form a hermetic seal. In an effort to address these problems, film coatings have been developed that provide improved heat-sealing characteristics, such as reduced sealing temperatures, for such thermoplastic films.
Current efforts in the hot seal packaging industry have been focused on developing thermoplastic films which provide good seal strength immediately after a hermetic seal is formed at increasingly lower temperatures and faster machine speeds in HFFS and VFFS equipment. One property in which to evaluate such hermetic seals immediately after seal formation is “hot tack.” Hot tack has been defined as the capability of a heat-seal joint to hang together when it is stressed, while still hot from the sealing operation, or more technically speaking, the sum of the cohesive strength of a sealant material as well as its adhesive strength to the remaining elements of the multilayer structure while in the heat-seal temperature range from an article entitled “Sealing in Brand Equity with Hot Tack”, by James R. De Garavilla, DuPont Packaging, dated 10 Sep. 2005 (http://www.dupont.com/packaging/knowledge/tech0004.html). Hot tack differs from “heat seal initiation temperature” which measures the temperature at which a heat seal begins to form with a minimum defined strength of such seal.
At increased machine speeds, hot seal packaging may compete with higher cost and more complex cold seal packaging. Innovations to date have been primarily concerned with coatings that form good hermetic seals at low temperatures when applied to thermoplastic films. While coatings based on ethylene and acrylic acid copolymers exhibit good low temperature sealing and hot tack properties, such coatings do not exhibit sufficiently good sealing characteristics to significantly increase machine speed to match cold seal coated film performance. While acrylic-based and polyurethane-based coatings exhibit good sealing, blocking and slip characteristics, such coatings often exhibit unacceptable hot tack properties.
Coating compositions comprised of a first polymer dispersed in a liquid and having a mean particle size of less than or equal to 5000 nanometers were coated onto substrates (e.g., multilayer films) and formed into packages. The hermetic seals produced exhibited good low temperature sealing properties, but lacked desired hot tack properties. Similarly, coating compositions comprised of a second polymer dispersed in a liquid and having a mean particle size of less than or equal to 50 nanometers were coated onto substrates (e.g., multilayer films) and formed into packages. The hermetic seals produced exhibited good low temperature sealing and hot tack properties, but lacked desired antiblocking properties.
Therefore, there is a need for coating compositions which when coated onto substrates (e.g., multilayer films) exhibit improved antiblocking characteristics. When the coated substrates are used to form hermetic seals, the coated substrates exhibit improved low temperature sealing properties. When the hermetic seals comprise a package for a product, the hermetic seals exhibit improved hot tack properties. The invention disclosed herein meets these and other needs.
Prior Patents and Publications
The following references may be pertinent to this invention disclosed herein.
U.S. Pat. No. 5,419,960 of Touhsaent of the Mobil Oil Corporation discloses low temperature seal coatings wherein a polymer film, such as oriented isotactic polypropylene, is coated with a composition comprising a copolymer of about 65 to 95 wt % ethylene and about 5 to 35 wt % of acrylic or methacrylic acid, based on the weight of the polymer, in which about 2 to 80% of the carboxylate groups are neutralized with metal ions from Group IA, IIA or IIB of the Periodic Table, preferably sodium ions.
PCT Publication WO 2005/026275 of Gringoire et al. of the ExxonMobil Oil Corporation discloses novel coated substrates wherein a substrate is coated with a coating composition that forms good hermetic seals at very low temperatures and exhibits good blocking and slip characteristics. Such coating composition is comprised of an aqueous dispersion of a substantially non-polar copolymer of ethylene and an alpha-olefin having less than 20 carbon atoms, and an additive selected from the group consisting of an anti-blocking and slip agent, a coalescent and wetting agent, and mixtures thereof.
U.S. Publication 2005/0100754 A1 of Moncla et al. of The Dow Chemical Company discloses aqueous dispersions comprising at least one thermoplastic resin, at least one dispersing agent, and water, wherein the dispersion has a pH of less than 12, including aqueous dispersions having a volume average particle size of less than about 5 microns (5000 nanometers). Some aqueous dispersions disclosed have an average particle size of less than about 1.5 microns (1500 nanometers) and in the range from about 0.05 to about 1.5 microns (50 to about 1500 nanometers). Disclosed thermoplastic resins include alpha-olefin interpolymers of ethylene with at least one comonomer selected from the group consisting of C4-C20 linear, branched or cyclic dienes, or an ethylene vinyl compound, and others. Disclosed thermoplastic resins also include alpha-olefin interpolymers of propylene with at least one comonomer selected from the group consisting of C4-C20 linear, branched or cycle dienes, and others. Disclosed are any suitable dispersing agents, including ethylene-carboxylic acid polymers, and their salts, such as ethylene-acrylic acid copolymers or ethylene-methacrylic acid copolymers, and others. It is disclosed that the dispersing agents may be combined and comprises less than about 4 wt % based on the weight of the thermoplastic resin. Also disclosed in US2005/0100754 is that the aqueous dispersions may be coated onto a substrate, including thermoplastic films such as polyethylene terephthalate, polyethlylene, and others.
PCT Publication WO 2005/085331 A1 of Moncla et al. of The Dow Chemical Company discloses aqueous dispersions comprising at least one thermoplastic resin, at least one dispersing agent, and water, wherein the dispersion has a pH of less than 12, including aqueous dispersions having a volume average particle size of less than about 5 microns (5000 nanometers). Some aqueous dispersions disclosed have an average particle size of less than about 1.5 microns and in the range from about 0.05 microns to about 1.5 microns. Disclosed thermoplastic resins include alpha-olefin interpolymers of ethylene with at least one comonomer selected from the group consisting of C4-C20 linear, branched or cyclic dienes, or an ethylene-vinyl compound, and others. Disclosed thermoplastic resins also include alpha-olefin interpolymers of propylene with at least one comonomer selected from the group consisting of C4-C20 linear, branched or cyclic dienes, and others. Disclosed dispersing agents include ethylene-carboxylic acid polymers, and their salts, such as ethylene-acrylic acid copolymers or ethylene-methacrylic acid copolymers, and others. The dispersing agent comprises less than about 4 wt % based on the weight of the thermoplastic resin.
Also disclosed in WO 2005/085331 A1 is a method for forming a heat sealable coating on a substrate, wherein the substrate is formed from at least one oriented polymer. Disclosed substrates include thermoplastic films, such as propylene homopolymers and random copolymers and oriented nylon. The method includes depositing an aqueous polymer dispersion on the substrate, wherein the aqueous polymer dispersion includes at least one thermoplastic resin, at least one dispersing agent, and water, wherein the dispersion has a pH of less than 12, and drying the dispersion to form a first layer. The dispersion has an average volume diameter particle size of less than about 5 microns. Disclosed are heat seal initiation temperatures of 80° C. or below, 75° C. or below, 70° C. or below, and 65° C. or below.